1. Field of the Invention
The present invention relates to a method of forming a temperature control structure for an integrated circuit, and in particular, to a temperature control structure fabricated on the surface of an integrated circuit that is capable of changing the heat distribution profile directly on the chip.
2. Description of the Related Art
As device sizes continue to shrink into the submicron region, packing densities have increased dramatically. One consequence of this heightened density is that the thermal energy generated by the operation of semiconducting devices is now a key factor that must be considered in designing a chip. Absent some form of heat management, heat from device operation could accumulate and degrade device performance.
FIG. 1 shows a schematic depiction of the Seebeck Effect. Due to this thermoelectric phenomenon, a Seebeck voltage (V.sub.S) will be generated on aluminum contacts Al.sub.1 and Al.sub.2 when these contacts are connected to a second conductor of a different material (silicon), and when the Al.sub.1 and Al.sub.2 contacts are maintained at different temperatures T.sub.1 and T.sub.2 respectively. The Seebeck voltage generated is proportional to the temperature difference (T.sub.1 -T.sub.2), with
V.sub.S =.alpha.(T.sub.1 -T.sub.2)
where .alpha. is the Seebeck coefficient. Room temperature values of .alpha. are typically in the range of -10.0 to +10.0 .mu.V/.degree. C. With Al/Si junctions, values of .alpha. may be as large as 1.4 mV/.degree. C., which is the same order of magnitude as the temperature coefficient of the base-emitter voltage (V.sub.be) of a bipolar transistor.
FIG. 2 shows a schematic depiction of the Peltier Effect. The Peltier Effect is simply the converse of the Seebeck Effect Under the Peltier Effect, current passed through junctions of dissimilar conductors (such as Al and Si) results in absorption of heat at one junction and the emission of heat at the other junction. Due to the frequent application of voltages to operate silicon type semiconductor devices having metal contacts, the Peltier Effect is perhaps the dominant thermal effect experienced by integrated circuits.
The Peltier and Seebeck Effects are related directly by ##EQU1##
Since II.sub.ab (Peltier) is defined per unit current flow, the power output (V*I) from a Seebeck junction pair with a small temperature difference .DELTA.T is related to the Peltier heat transfer by a factor of the form .DELTA.T/T.
In considering FIG. 2, it is important to note that the relative temperatures of T.sub.1 and T.sub.2 are directly related to the polarity of the voltage drop applied across the aluminum contacts. Therefore, reversing the polarity of the voltage source would make T.sub.1 less than T.sub.2.
It is also important to note that changing the conductivity type of the silicon bridge in FIG. 2 will also have the effect of reversing the relationship between T.sub.1 and T.sub.2. Thus, where the Si bridge is N-type rather than P-type as shown, T.sub.1 would also be less than T.sub.2.
The Peltier Effect has previously been used to manage heat generated on IC structures. FIG. 3 shows such a conventional Peltier structure as used to cool an IC present on a chip.
Conventional Peltier structure 300 includes monolithic blocks of N- and P-type conductivity type 304 and 306, respectively. N-type block 304 and P-type block 306 are typically composed of different Tellurium/Bismuth compounds. For example, BiSb.sub.4 Te.sub.7.5 exhibits P-type properties, while Bi.sub.2 Te.sub.2 Se is an N-type material.
Second end 304b of N-type block 304 and second end 306b of P-type block 306 are electrically connected by metal strip 308. First end 304a of N-type block 304 and first end 306a of N-type block 306 are connected to metal contacts 310 and 312, respectively.
As described in detail in connection with FIG. 2 above, application of a potential difference across contacts 310 and 312 results in a temperature change at metal strip 308, thus and on chip 302 in contact therewith. Specifically, as shown in FIG. 3 application of a negative voltage difference across N block contact 310 and P block contact 312 results in a temperature increase at contacts 310 and 312, and a corresponding temperature decrease at metal strip 308. Conversely, application of a positive voltage difference across N block contact 310 and P block contact 312 would result in a fall in the temperature of contacts 310 and 312 and a corresponding rise in the temperature of metal strip 308.
As is evident from the above description in connection with FIG. 3, application of a voltage difference to the conventional Peltier structure can result in temperature control of a chip. However, it is important to note that this conventional Peltier structure suffers from a number of serious disadvantages.
First, the temperature control exercised over the chip is crude and non-specific. In particular, the monolithic P- and N-type blocks making up the conventional Peltier structure are far larger than the individual semiconducting structures present on the chip. Application of a voltage to the conventional Peltier structure therefore necessitates a temperature change over the entire chip, and all component structures of an integrated circuit present thereon. Temperature control of specific structures of the integrated circuit, or even of selected regions of the chip, is not possible with the conventional Peltier structure.
Therefore, there is a need in the art for a temperature control device which enables highly specific and selective management of heat on an IC circuit present on a chip.
A second disadvantage associated with the conventional Peltier cell structure is the substantial power consumption of the device. A current in the range of between 3 and 5 Amp would typically be required to maintain a desired temperature change across the monolithic P and N type blocks shown. This is due in part to the sheer mass of the P- and N-type blocks of the Peltier cell, which dwarf any individual semiconducting structures present on the chip.
Therefore, there is a need in the art for a temperature control device which consumes relatively little power while effectively managing the temperature of an IC.
A third disadvantage associated with a conventional Peltier cell is the space consumed by the device. As shown in FIG. 3, transfer of thermal energy to and from the chip is accomplished in a direction out of the plane of the chip. This orientation consumes precious space present around the chip. This space is generally quite limited, due to the reduced size of portable applications such as laptop computers, as well as the necessary presence of nearby components such as circuit boards, chip packaging, buses, or other forms of communication structures.
Therefore, there is a need in the art for a temperature control device which occupies relatively little space and which can be incorporated into an IC device directly upon the surface of a semiconducting chip.